(tŏm'bô') American astronomer who discovered the planet Pluto in 1930. The existence of an outermost planet, known as Planet X, had been predicted some years earlier by the American astronomer Percival Lowell based on apparent perturbations in the orbits of Neptune and Uranus. Tombaugh's careful analysis of celestial photographs did reveal a new planetary body, although it was too small to have caused the perturbations. Planet X itself was never found.